Process for the preparation of 4,4'-dimethyl-6,6'-dichlorothioindigo

Abstract

4,4'-Dimethyl-6,6'-dichlorothioindigo is obtained in the stable crystal modification I, distinguished by the fraction angles (2 theta): lines of high intensity: 25.99; 27.26 lines of moderate intensity: 10.27; 12.48; 22.39; 23.91; 24.82; 28.87, by oxidizing 3-hydroxy-4-methyl-6-chlorothionaphthene having a mean particle size of at most 20 μm.

We claim: 
     
       1. A process for the preparation of 4,4'-dimethyl-6,6'-dichlorothioindigo of the formula (I) ##STR6## in the stable crystal modification I, which, in the X-ray diffraction pattern measured with CuK.sub.α -radiation, has lines at the following diffraction angles 2 θ (°): lines of high intensity: 25.99; 27.26   lines of moderate intensity: 10.27; 12.48; 22.39; 23.91; 24.82; 28.87,   wherein 3-hydroxy-4-methyl-6-chlorothionaphthene of the formula (II) ##STR7##  has a mean particle size D 50  of at most 20 μm and is oxidized using an oxidizing agent to give the pigment of the formula I.   
     
     
       2. The process as claimed in claim 1, wherein the compound of the formula (II) has a mean particle size D 50  of at most 15 μm. 
     
     
       3. The process as claimed in claim 1, wherein the oxidizing agent is an iron(III) salt, a nitro compound, oxygen, ozone, percarbonate, a perborate or a polysulfide. 
     
     
       4. The process as claimed in claim 1, wherein the oxidizing agent is an alkali metal polysulfide. 
     
     
       5. The process as claimed in claim 1, wherein the oxidizing agent is disodium polysulfide. 
     
     
       6. The process as claimed in claim 1, wherein the oxidation is carried out in the presence of a surfactant or a surfactant mixture. 
     
     
       7. The process as claimed in claim 6, wherein the surfactant is a nonionogenic surfactant. 
     
     
       8. The process as claimed in claim 7, wherein the nonionogenic surfactant is an ethoxylation product of a fatty amine, of a fatty amide, of a fatty alcohol, of a fatty acid, of an alkylphenol or of a polypropylene glycol. 
     
     
       9. The process as claimed in claim 1, wherein the oxidation is carried out at a temperature of from 30 to 150° C. 
     
     
       10. The process as claimed in claim 1, wherein the oxidation is carried out at a temperature of from 50 to 100° C. 
     
     
       11. The process as claimed in claim 1, wherein the oxidation is carried out in a mixture of water and one or more organic solvents which are partially or completely miscible with water. 
     
     
       12. The process as claimed in claim 1, wherein the oxidation is carried out in water.